1. Field of the Invention
The present invention relates generally to an apparatus for controlling flow of gas and water through a fuel cell, and more particularly to an asymmetrical gas diffusion layer arrangement with an anode gas diffusion layer with a higher diffusion resistance than the cathode gas diffusion layer.
2. Background Art
Fuel cells are used as an electrical power source in many applications. In particular, fuel cells are proposed for use in automobiles to replace internal combustion engines. In proton exchange membrane (“PEM”) type fuel cells, hydrogen (H2) is supplied as fuel to the anode of the fuel cell, and oxygen is supplied as the oxidant to the cathode. The oxygen can either be in pure form (O2) or air (a mixture of O2, N2, CO2, and other gases). Proton exchange membrane fuel cells typically have a membrane electrode assembly (“MEA”) in which a solid polymer membrane has an anode catalyst on one face, and a cathode catalyst on the opposite face. The MEA is sandwiched between a pair of porous gas diffusion layers (“GDL”), which in turn are sandwiched between a pair of non-porous, electrically conductive elements or plates. These plates function as current collectors for the anode and the cathode, and contain appropriate channels and openings formed therein for distributing the fuel cell's gaseous reactants over the surfaces of the respective anode and cathode catalysts. In order to produce electricity efficiently, the polymer electrolyte membrane of a PEM fuel cell must be thin, chemically stable, proton transmissive, non-electrically conductive and gas impermeable. In typical applications, fuel cells are provided in arrays of many individual fuel cell stacks in order to provide high levels of electrical power.
Gas diffusion layers play a multifunctional role in proton exchange membrane fuel cells. For example, GDLs act as diffusers for reactant gases traveling to the anode and the cathode layers, while transporting product water to the flow field. GDLs also conduct electrons and transfers heat generated at the MEA to the coolant, and acts as a buffer layer between the soft MEA and the stiff bipolar plates. Among these functions, the water management capability of GDL is critical to enable the highest fuel cell performance. In other words, an ideal GDL would be able to remove the excess product water from an electrode during wet operating conditions or at high current densities to avoid flooding, and also maintains a certain degree of membrane electrolyte hydration to obtain decent proton conductivity during dry operating conditions. The solid electrolyte membrane (such as Nafion®) used in proton exchange membrane fuel cells needs to maintain a certain degree of hydration to provide good proton conductivity. Hydrocarbon based proton exchange membranes, which are emerging as an alternative solid electrolyte for fuel cell applications, have the potential to be cheaper and more favorable (no fluorine release) compared to the fluoropolymer-based solid electrolyte membrane such as Nafion. The hydrocarbon-based solid electrolyte membranes developed to date need a higher degree of hydration in order to achieve decent proton conductivity.
For PEM fuel cells targeting automotive applications, a drier steady state operating condition is favorable, which requires good water retention capability of the GDL to maintain a certain degree of membrane hydration. The fuel cells in automotive applications will also experience wet operating conditions during start up, shut down and in a subfreezing environment.
Many years of research regarding materials that are thin, porous and conductive has resulted in the polyacrylonitrile (“PAN”)-based carbon fiber paper used in state-of-the-art PEM fuel cells. However, attempts to use alternate lower cost materials to PAN-based carbon fiber paper has resulted in one or more of the gas diffusion layer functions being adversely affected. One problem which arises when the gas diffusion layer does not function optimally is anode water accumulation which can cause freeze and cold start failures in the current modules. Increasing the anode diffusion resistance can impact water balance and help lessen the occurrence of freeze and cold start failures. However, in the past these low-cost materials were generally considered symmetrically by evaluating performance with the same materials on both the cathode and anode sides of the fuel cell.